Linear measurement systems are used in the case of a linear position determination, in particular in the case of high-precision applications in the field of precision machines, coordinate measuring machines, or comparable fields of application. In some applications, measurement accuracies in the sub-millimeter range or sub-micrometer range are necessary in this case, in other applications, machines, or devices, in which the present invention can also be used, the accuracy requirements can also be in the millimeter range, however. In many of these applications, the linear measurement systems are preferably designed as absolute value linear position encoders, which provide an unambiguous, absolute position value over the entire length of the measurement range thereof.
For example, EP 0 268 558 discloses an absolute scale, which is divided into two separate, parallel scales having periodic coding of different periodicity.
DE 38 18 044 discloses a use of multiple line sensors on a subcarrier, which preferably has a coefficient of thermal expansion of zero or equal to that of the scale. The coding is designed as an interleaving of absolute and incremental coding in a shared scale.
In a coordinate measuring machine in gantry construction, for example, numerous linear sensors are used to determine the spatial position of a measurement head in multiple dimensions. In this case, the measurement ranges to be acquired can certainly be multiple meters. Therefore, appropriate absolute-coded linear measurement systems are to be provided, using which an absolute position value of a reading head in relation to a code scale is ascertainable with sufficient position resolution, and using which the entire measurement range can be absolute-coded, even with large measurement ranges. In this case, the ability to read out and analyze the code is to be kept as simple as possible.
In the case of position accuracies in the micrometer range or less, in addition to the movements in the longitudinal direction of the linear sensors, deviations in the transverse direction can also have an interfering effect on the accuracy of the ascertained spatial position of a measurement head. An effort is therefore to be made in particular to also make such parasitic effects ascertainable, for example, to be able to recognize measurement errors arising therefrom and/or compensate for them appropriately. Linear position encoder systems are thus preferably to be provided for such embodiments, using which, in addition to the primary absolute position determination thereof in the longitudinal direction along the linear encoder system, a location is determinable in a further direction deviating from the primary linear encoder measurement direction, although with a significantly restricted measurement range in relation to the primary linear measurement direction of the linear encoder, for example, with a measurement range of several millimeters or less. Thus, in an advantageous embodiment, in addition to the primary absolute position determination in the scale longitudinal direction, a determination of deviations in the transverse direction is to be sought at least in a restricted manner, for example.
For example, EP 0 042 179 discloses a system for determining positions along an advance direction, which especially enables a determination of deviations of an ideal location on the basis of a pattern having V-shaped strips. In this case, a position which is incrementally determined on the basis of a pattern is absolutely located by a further separate absolute pattern, which extends in parallel to the first pattern in the advance direction, wherein the absolute pattern codes coarse positions by means of a gray code.
DE 197 32 398 discloses the measurement of pivoting in both the X and Y directions. In addition, it describes boundary conditions for the coding.